While inkjet printing methods are well known for printing text and images meant for viewing by the human visual system, the ability of inkjet methods to produce patterns of functional fluids on a variety of substrates affords the possibility of expanding the use of inkjet methods into the creation of functional patterns such as electrical circuits, sensors, or even antimicrobial coatings. However, the use of inkjet methods for functional patterning presents a number of challenges related to the limitations of the ink jet printing hardware and the unique requirements needed for ejecting inks. For example, most inkjet printing methods are limited to a single drop size of greater than about 10 μm, and more typically of from 20 μm to 50 μm, thus limiting the linewidth of a functional pattern to at least those dimensions or greater to achieve robust performance. A variety of circuitry can be designed with large macroscopic linewidths that are well suited to inkjet printing and various methods of patterning could be combined to cover a wide range of linewidths.
Each type of ink jet ejection method places specific demands or limitations on the ink jet ink composition that can greatly limit the choice of materials used to formulate the ink jet ink composition. Continuous ink jetting requires the ink jet ink composition to be pumped at high pressure through a recirculation system that can include the use of high shear pumping and filtration that can damage or destabilize dispersions of polymers, pigments, or other functional materials such as metal nano-particles. Piezoelectric drop-on-demand (DOD) print heads are often the most tolerant of a wide range of ink jet ink composition formulations including non-aqueous, high viscosity, and even heat-curable formulations. Unfortunately, they tend to be expensive to manufacture and suffer from slower ink jet ejection rates. Thermal DOD print heads are relatively inexpensive to manufacture, offer a wide range of designs, and can be fired at ejection rates in excess of 30 kHz, thus allowing the highest print speed for DOD printing. They also require the ink jet ink composition to come into contact with a resistive heater that vaporizes enough ink jet ink composition to form a vapor bubble capable of ejecting the remaining ink jet ink composition in the chamber out through the nozzles such that the ejected drop typically has a velocity of about 10 m/sec or greater. The resistive heating element typically reaches temperature of about 300° C. that can decompose or destabilize the ink jet ink composition and cause kogation (build-up of decomposition products on the heater surface or in the ink chamber) and eventual nozzle clogging or heater failure.
The need for efficient energy transfer from the resistive heater to the ejected drop also favors aqueous-based inks due to the high vaporization energy of water, although non-aqueous thermal inks are known but rarely used. All DOD ejection systems will tend to form a very fine mist due to the formation of unwanted satellite drops. The use of organic solvents in ink jet ink compositions can create a serious health and safety hazard due to this fine mist formation.
Ink jet ink compositions used for functional printing (that may include electroless metal plating) will often be designed to adhere to flexible substrates such as polyesters like polyethylene terephthalate (PET) with good adhesion and durability. Ink jet ink compositions that can be hardened or cured using heat or radiation are well known and widely used for this purpose. They are often generically referred to as “UV-curable inks.” Such inks are typically formulated with vinyl monomers or oligomers that contain one or more pendant vinyl groups that can polymerize and form a crosslinked network. The polymerization or crosslinking is most often initiated by free radicals formed from an added initiator compound that decomposes into active radicals upon exposure to appropriate wavelength UV-radiation or heat. UV curing may occur almost simultaneously with the printing, or it can be delayed. UV-curable inks are most commonly organic solvent-based and are ink jet printed with piezo print heads to avoid the need for vaporization and the possibility of premature polymerization in the print head from thermal initiation by the high temperatures of the resistive heater of a thermal print head. Some aqueous ink jet ink compositions have been reported and contain water-soluble cross-linkable oligomers and free radical initiators such as those commercially available as Sartomer® SR415 and Irgacure® 2959.
UV-curable ink jet ink compositions formulated with multi-functional vinyl or acrylate monomers and UV-activated free radical initiators have several drawbacks, especially if they are ejected with a thermal printhead. The radical initiators, while triggered by UV light at temperatures typically below 100° C., will thermally decompose to form radicals at higher temperatures such as those near the 300° C. resistive heater in a thermal ejector chamber. This can cause premature polymerization in the nozzle or ink-feed plenum resulting in ejector plugging or failure and poor reliability. Secondly, the organic solvents and the vinyl monomers contained in the non-cured ink are flammable and often unsafe for human exposure, especially as an extremely fine mist that often accompanies most DOD printing. While some aqueous formulations are possible, the formulation options are limited due to limited solubility of the components and even aqueous-based formulations can still be hazardous because of the presence of acrylate monomers or oligomers known to cause toxic sensitization reactions on exposure.
Ink jet ink compositions useful for functional patterning to form catalytic patterns useful for further chemical processes such as electroless metal plating or sensor formation should be insoluble in aqueous solutions that are often strongly basic solutions after ink jet printing, but they should remain permeable to water and other aqueous reactants such as metal ions or complexes and reducing agents.
Thus, a unique type of UV-curable polymer is needed to avoid all of these problems. Such UV-curable polymers should be very water-soluble before crosslinking, but should become water-insoluble but still aqueous permeable after crosslinking and after the ink jet ink compositions are printed. The UV-curable polymers should also have the capability to complex metal ions and possibly stabilize the formation of metal nanoparticles by containing an adequate level of metal complexing functionality such as carboxylic or carboxylate groups, sulfonic acid or sulfonate groups, phosphonic acid or phosphonate groups, and possibly other functionality such as amide, alcohol, and amine groups. The crosslinking functionality of these polymers should also be stable to the thermal ejection process during ink jet printing and have typical long-term storage properties.
Aqueous UV-curable ink jet ink compositions that contain a multi-functional water-soluble acrylate monomers with free radical photoinitiators have been reported in U.S. Pat. No. 6,846,851 (Nakhmanovich et al.) and U.S. Patent Application Publication 2002/0198289 (Gummeson). This type of chemistry is also well known for the preparation of hydrogels and interpenetrating networks that can be used to complex and form stable metal nanoparticles. Unfortunately, these types of ink jet ink compositions will likely exhibit the drawbacks described above for UV-curable inks, especially when used for thermal print head ejection.
Thus, there is a need to address these problems with improved UV-curable ink jet ink compositions.